Polytetrafluoroethylene (PTFE) articles have been expanded by several methods to produce highly porous articles, with small pore sizes. U.S. Pat. No. 3,664,915 discloses the method of stretching articles of the fine powder type of PTFE to produce porous films in which porosity is defined by nodes and fibrils, the nodes being at least 1000.times. thicker the fibrils, and the pore size is at least 0.2 micrometers. Even smaller pore sizes, as low as 0.01 micrometers, have been reported in U.S. Pat. No. 4,110,392. PCT 93/12332 discloses the method of swelling PTFE, either the fine powder or the granular types, such as in billet form, with such fluids as organic or inorganic liquids or such gases as ammonia, nitrogen, sulfur dioxide, and carbon dioxide. The swelling step is carried out under pressure and at elevated temperature, and this is followed by slow, controlled cooling of the swollen article. Following the cooling step, the pressure is then released and the fluid is then removed from the PTFE, yielding a porous (swollen) PTFE structure having a void content of 15 to 35% and consisting of a mass of interconnecting fibrils. Stretching of the swollen article increases the void content to at least 40% and reduces pore size to be less than 10 micrometers and as low as 0.1 micrometers.
Melt-fabricable fluoropolymers have been foamed as they are fabricated, e.g. by melt extruding them in the presence of a blowing agent, either contained in the fluoropolymer feed to the extruder or a gas injected into the fluoropolymer within the extruder, whereupon the extrudate foams as it is extruded. The fluoropolymer may contain a foam cell nucleating agent such as disclosed in U.S. Pat. No. 5,023,279, to produce smaller foam cells and higher void content. The disadvantage of this approach is that foaming during extrusion favors the use of low melt viscosity resin instead of higher melt viscosity resin which exhibit higher melt strength and better physical properties for the extrudate.
There is a need for the ability to foam PTFE with efficiency and economy and for the ability to foam melt-fabricable fluoropolymer after fabrication so that the limitations on fabrication imposed by the foaming process are not present.